Chapter 6. BJT Amplifiers

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1 Basic Electronic Devices and Circuits EE 111 Electrical Engineering Majmaah University 2 nd Semester 1432/1433 H Chapter 6 BJT Amplifiers 1

2 Introduction The things you learned about biasing a transistor in Chapter 5 are now applied in this chapter where BJT circuits are used as small-signal amplifiers. The term small-signal refers to the use of signals that take up a relatively small percentage of an amplifier s operational range. Additionally, you will learn how to reduce an amplifier to an equivalent dc and ac circuit for easier analysis. 2

3 AC Quantities AC quantities are indicated with a lowercase italic subscript; V rms: 1/ avg: 2/π rms values are assumed unless otherwise stated. The figure shows an example of a specific waveform for the collector-emitter voltage. Notice the DC component is V CE and the ac component is V ce. rms avg V CE 0 0 v ce V ce V ce V ce V ce t Resistance is also identified with a lowercase subscript when analyzed from an ac standpoint. 3

4 Linear Amplifier A linear amplifier produces a replica of the input signal at the output. +V CC I c V b I CQ V BQ R 1 R C V ce C 2 R s C 1 I b V CEQ I BQ V s R 2 R E R L For the amplifier shown, notice that the voltage waveform is inverted between the input and output but has the same shape. 4

5 AC Load Line V CC /(R C R L +R E ) I C I b I BQ I c Operation of the linear amplifier can be illustrated using an ac load line. The ac load line is different than the dc load line because a capacitor looks open to dc but effectively acts as a short to ac. Thus the collector resistor R C appears to be in parallel with the load resistor R L. I CQ Q 0 V CE V ce V CEQ ac ground ac short; Z C = 1/( j ω C ) 5

peak-to-peak value of 6 4 = 2 ma and the collector-to-emitter

6 Solution: Projections on the graph of Figure 6 4 show the collector current I c varying from 6 ma to 4 ma for a peak-to-peak value of 6 4 = 2 ma and the collector-to-emitter voltage V ce varying from 1 V to 2 V for a peak-to-peak value of 2 1 = 1 V. 6

7 Transistor AC Model very large very small 7

8 8

Transistor AC Model The five resistance parameters (r-parameters) can be used for detailed analysis of a BJT circuit. For most analysis work, the simplified r-parameters give good results.

9 Transistor AC Model The five resistance parameters (r-parameters) can be used for detailed analysis of a BJT circuit. For most analysis work, the simplified r-parameters give good results. The simplified r-parameters are shown in relation to the transistor model. An important r-parameter is r e '. It appears as a small ac resistance between the base and emitter. ' r e 25 mv = I assuming an abrupt junction between the n and p regions, and an ambient temperature of 20 C E 9 B C E r e β ac I b B I b C r e E β ac I b

10 Quiz Q. The equation for finding the ac emitter resistance of a BJT is a. b. c. d. ' r e r ' e r ' e r ' e 25 mv = I B E B 25 mv = I E 0.7 V = I 0.7 V = I 10

11 I E = 25 mv / 8 Ω = ma 11

12 Comparison of β ac to β DC Curve of I C versus I B is nonlinear! slope slope 12

13 h (hybrid) parameters 13

14 Relationships of h Parameters and r Parameters forward current gain, common-base forward current gain, common-emitter voltage feedback ratio, common-emitter output admittance (conductance), common-emitter input impedance (resistance), common-emitter 14

Electronic Devices. Floyd. Chapter 6. Ninth Edition. Electronic Devices, 9th edition Thomas L. Floyd

Electronic Devices. Floyd. Chapter 6. Ninth Edition. Electronic Devices, 9th edition Thomas L. Floyd Electronic Devices Ninth Edition Floyd Chapter 6 Agenda BJT AC Analysis Linear Amplifier AC Load Line Transistor AC Model Common Emitter Amplifier Common Collector Amplifier Common Base Amplifier Special